1. Field of the Invention
The present invention relates to a capillary holding structure for an ultrasonic horn used in a wire bonding apparatus.
2. Prior Art
In a wire bonding apparatus, as is universally known, an ultrasonic horn which holds a capillary at one end thereof is raised and lowered and also moved in the horizontal X and Y directions, and a wire that passes through the capillary is connected (bonded) between, for instance, the electrodes of semiconductor pellets and the lead posts of external leads.
Examples of conventional capillary holding structures for ultrasonic horns are shown in FIGS. 5 and 6; and Japanese Utility Model Application Laid-Open (Kokai) No. H5-59840 and Japanese Patent Application Laid-Open (Kokai) No. H6-163648 disclose such capillary holding structures for ultrasonic horns.
In the structure of FIG. 5, a capillary attachment hole 3 which holds a capillary 2 therein is formed in one end of an ultrasonic horn 1, and a longitudinal slit 4 is formed, as best seen from FIG. 5(A), so as to extend from the tip end (left end in FIG. 5) of the ultrasonic horn l and cross the capillary attachment hole 3. The capillary 2 is held in the ultrasonic horn I when a bolt 5 that passes through the slit 4 is tightened.
In the structure of FIG. 6, as shown in FIG. 6(B), the capillary attachment hole 3 of a constant inner diameter is not opened through the entire height of the ultrasonic horn for the constant diameter; instead, a stepped hole 3a which is for positioning the upper end of the capillary 2 is formed in the attachment hole 3. A longitudinal slit 4 is formed so as to extend from the tip end of the ultrasonic horn 1 and cross the capillary attachment hole 3, so that the area surrounding the capillary attachment hole 3 is endowed with an elastic force. Thus, the capillary 2 is held in the ultrasonic horn 1 by this elastic force alone without using a bolt. The reference numeral 6 in FIG. 6 is a threaded hole used for accomplishing a screw engagement with a bolt (not shown) that is used when replacing the capillary 2. When the bolt (not shown) is screw engaged with the threaded hole 6, the slit 4 is opened so that the capillary 2 can be released.
In either of the above structures, since the slit 4 extends from the tip end of the ultrasonic horn 1 towards the root portion thereof, the tip end part of the ultrasonic horn 1 is vertically bifurcated and has an open end 4a. As a result, when the bolt 5 is tightened down in the structure of FIG. 5, the open end 4a spreads outward, and the tightening force of the bolt 5 escapes, reducing the holding force. This would cause the capillary 2 to be unsteady or to fall out during the bonding process. On the other hand, in the structure shown in FIG. 6, the capillary 2 is held only by the elastic force of the capillary attachment hole 3, and the ultrasonic horn 1 has an open end 4a. Accordingly, there is a conspicuous deterioration in the holding force over time, and therefore a stable holding force cannot be maintained over a long period of time.